Trauma: The types of trauma are endless but we divided them into several categories. Either 
the end result is osteoarthritis ( she is an older dog ) leading to a degenerative joint and 
subsequent cruciate rupture or prior trauma that weakened structures/predisposed to injury 
and osteoarthritis;

1. Except for acute trauma, cranial cruciate ligament failure due to advancing age( 
primary stimulus)appears to be  a gradual process during which the ligament stretches, 
probably tears and then eventually fails.  Early in the process of failure,  the joint will 
become unstable and osteoarthritic change is initiated. The clincal problem may not 
become evident until the ligament completely fails and by then the osteoarthritis is well 
advanced.  Some cases may present with lameness before total failure.  Eventual 
total failure is associated with MILD TRAUMA, not the type of injury 
expected to tear a normal ligament. The reverse may happen as well where there is 
primary osteoarthritis( changes in the articular surface) which causes changes in the 
cruciate . Although acute cruciate ligament rupture does occur, it is thought the majority 
of lesions are the result of chronic degenerative changes in the ligaments themselves.  
Repeated minor stresses can result in progressive DJD of the stifle. These are 
frequently bilateral. These stresses are compounded in the obese animal.  As joint 
changes develop, the cruciate ligaments begin to degenerate and undergo alterations in 
their microstructure.  The collagen fibrils become hylanized and the tensile strength of 
the ligaments is reduced, making the ligaments more susceptible to damage from 
minimal trauma. These changes have been associated with the aging process and may 
explain the fact that most cruciate injuries are seen in animals older than 5 years.

2. The cruciate tear could also be concurrent with other types of injuries such as an 
avulsion of the proximal tendon of the long digital extensor muscle.  While this usually 
happens in large breed immature animals, it can happen in the adult as well.  The LDE 
muscle originates in the extensor fossa of the lateral femoral condyle. The tendon 
crosses the joint and passes deep to the cranial tibial muscle thru a sulcus in the 
proximolateral  tibia.  It is apparently NOT important in the stability of the stifle joint 
but can cause pain and effusion immediately after injury. If left untreated, then 
osteoarthritic changes can ensue.  Likewise, if there was any injury to the joint that was 
not detected at an earlier point in BonnieÕs life (including partial tears), that could have 
set the stage for osteoarthritis and eventually cruciate ligament failure.

3. BonnieÕs early history is suspect since she was not found until 6 months of age and not 
adopted until 8 months of age. There are several situations that may have contributed to 
her current state. First, she may have had osteochondrosis as a puppy but it resolved 
spontaneoulsy before the owner or the humane society ever saw signs of lameness. 
Unfortunately, this predisposes animals to osteoarthritic changes earlier in life so 
Bonnie may have osteoarthritis secondary to that which led to CCL tear.  Additionally, 
Bonnie may have conformational problems that are not bad enough to cause clinical 
signs but may have led to chronic abnormal forces on the articular cartilage of the stifle 
joint leading to osteoarthritis. Such abnormalities include hip dysplasia (not always 
evident clinically but may see signs radiographically), and deformity of the proximal 
tibia with caudal bowing.  

4. Bonnie may have also been exposed to some sort of previous trauma as an abandoned 
puppy such as being hit by a car, thrown out of car, hit by a person, dropped off 
tables, etc. We have no idea if there may have been a previous hairline fracture where 
the lamenss again may have resolved but also set the stage for osteoarthritis.

5. Acute traumaÑany type of excessive outside force or torsion may have caused rupture 
of the cruciate ligament without any predisposing factors but this is rather unlikely 
considering the dogÕs age and weight and the fact that the dog is usually with the 
owner. It is possible that Bonnie got away to chase another dog or a cat and injured 
herself in the process or she may have injured herself while alone in the house, but the 
likelihood of trauma being the only cause of the ruptured cruciate is minimal.

Osteoarthritis pathophysiology:
Damage to the chondrocyte or cartiliage matrix due to ANY NUMBER OF STIMULI (see 
above) causes an increase in the anabolic and catabolic activity of chondrocytes.  This 
includes increased production of metalloproteinases as well as release of inflammatory 
mediators such as cytokines and prostaglandins.  Synovial macrophages clean up the debris 
from the breakdown of proteoglycan and collagen and there is increased activity of 
synoviocytes.  The cartilage damage can result in release of cytokines and the catabolic 
activity eventually exceeds the anabolic activity resulting in depletion of cartilage matrix and 
the start of cartilage degradation. Proteoglycans are lost and cannot be replaced fast 
enough. The abnormal cartilage is now unable to withstand normal biochemical forces and 
this extra force will do further damage to the chondrocytes and matrix.  The subchondral 
bone begins to thicken and leads to sclerosis of the tissue.   Eventually the bone remodels. 
The synovial vasculature becomes more permeable in response to injury and inflammation, 
resulting in increased fluid accumulation. The fibrous joint capsule experiences fibroplasia 
and increased vascular stability.